Processing of Metal Values from Concentrates

ABSTRACT

The present invention relates to an improved method for the recovery of metal values, in particular copper and gold, from a metal value-bearing material containing arsenic and/or antimony and a source of sulphate ions, by means of a high temperature pressure oxidation process followed by cyanidation of the resultant high temperature pressure oxidation residue.

FIELD OF THE INVENTION

The present invention relates to an improved process for the recovery ofmetal values, in particular copper and gold, from metal-bearingconcentrates.

The present invention also relates more particularly to an improvedprocess for the recovery of metal values, in particular copper and gold,from metal-bearing concentrates by means of a high temperature pressureoxidation process followed by cyanidation of the resultant hightemperature pressure oxidation residue.

The present invention also relates. more particularly but notexclusively to a process of maximising copper and gold extraction frommetal-bearing concentrates that also contain significant amountsof-arsenic and/or antimony, and that substantially simultaneouslyresults in the formation of environmentally stable iron-arsenic and/oriron-antimony compounds in the process residues that can be dischargedto tailings dams or the like such that strict environmental regulationsare complied with.

The present invention also relates more particularly but not exclusivelyto a high temperature pressure oxidation process in which there iscontrolled oxygen addition to the first compartment of a pressure vesselsuch as a substantially continuously operated autoclave, and also moreparticularly relates to controlled oxygen addition to approximately thefirst 50% of the total volume of the continuously operated autoclave.

The present invention also relates more particularly but not exclusivelyto a high temperature pressure oxidation process in which the OxygenReduction Potential (ORP) of the reaction slurry in the firstcompartment, and typically approximately the first 50% of die totalvolume, of a pressure vessel such as a substantially continuouslyoperated autoclave, is kept below about 425 mV, and preferably belowabout 400 mV, when measured with a standard platinum (Pt) electrodeagainst a standard silver/silver chloride (Ag/AgCl) electrode, and thesoluble ferric to ferrous molar ratio is below about 1:1. The ORP andferric and ferrous iron assays referred to above are those obtained byrapid cooling to room temperature of a sample of slurry withdrawn fromthe autoclave within one hour and then filtered for assay purposes.

BACKGROUND OF THE INVENTION

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date:

(i) part of common general knowledge; or(ii) known to be relevant to an attempt to solve any problem with whichthis specification is concerned.

Many base metals are. sourced from sulphide ores. For example, coppersulphide minerals such as chalcopyrite [CuFeS₂] contribute to themajority of global copper production. There are also many other depositsthat contain copper in the form of arsenic-bearing minerals, primarilyenargite [Cu₃AsS₄] and tennantite [Cu₁₂As₄S₁₃], and/or antimony-bearingminerals such as tetrahedrite [Cu₁₂Sb₄S₁₃]. Included in such deposits isthe enargite-containing copper-gold resource at Chelopech, Bulgaria.

Processes that involve the recovery of metal values fromarsenic-containing sulphide minerals such as those indicated abovegenerally require consideration of the form or forms in which thearsenic component reports, and the environmental impact upon thedisposal of such arsenic-containing residues. Relevant national andinternational discharge regulations specify the maximum allowabledissolution of arsenic from such arsenic-containing residues underappropriate disposal regimes.

Pyrometallurgical treatment of arsenic-containing metal sulphideminerals is generally regarded as technically and economicallyundesirable, as most of the arsenic reports as a flue dust and as aspeiss phase. Safe disposal of these arsenic-containing materialsinvolves considerable cost and technical disincentives.

By contrast, many hydrometallurgical processes for treating coppersulphide minerals that also contain arsenic are directed teds thegeneration of an acidic copper sulphate solution containing solublecopper, which is typically recovered therefrom by a combination ofsolvent extraction and electrowinning. The arsenic component of the feedmaterial is converted into an insoluble arsenic-containing phase such ashydrated ferric arsenate [FeAsO₄2H₂O]. This particular phase also occursin nature as the mineral scorodite. The hydrated ferric sulphateproduced by the hydrometallurgical processes can be safely disposed ofin a conventional tailings impoundment. Most of the hydrometallurgicalprocesses for treating copper sulphide minerals generally fall withinthe general designation of pressure oxidation processes.

The kinetics of the copper leaching stage of many such pressureoxidation processes are frequently slow and there is generallyco-precipitation of an iron-capper-arsenate-sulphate compound orcompounds, leading to copper losses to the leach stage solid residue andthus to the overall process. Various means have been proposed toovercome the slow leach kinetics, including finer grinding of the feedmaterial, although these sometimes result in substantially increasedcapital and operating costs.

Many copper sulphide materials that contain arsenic and/or antimonyoften also contain metal values including precious metal values such asthose of gold and/or silver, and any process to treat such materialsmust also employ economically viable treatment stages to recover thegeld and/or silver contents. In the pressure oxidation process describedabove, the gold and/or silver generally report to the solid residuegenerated by the leach process. The gold and/or silver are usuallyrecovered by repulping the residue and cyanide leaching under theappropriate alkaline pH conditions. Meta-stable iron compounds such asbasic ferric sulphate [Fe(OH)SO₄] and any copper-containing precipitatessuch as an iron-copper-arsenate-sulphate in the residue will decompose(break down) under the alkaline pH conditions required for gold/silvercyanidation and thus bring about an increase in the lime and cyanideconsumption, thereby decreasing the economic efficiencies of the overallprocess. In other words, the abovementioned solid components present inthe leach residue break down during the cyanidation step, generatingexcess acid and reactive sulphate compounds that must be subsequentlyneutralised.

In summary, many of the hydrometallurgical processes currently employedto treat arsenic- and/or antimony-containing copper sulphide materialssuffer from unacceptable copper losses to the leach residue. Moreover,if the feed material also contains precious metal values such as thoseof gold and/or silver, then current processing conditions also lead tothe generation of solid residues that result in unacceptably high limeand cyanide consumption.

The present invention seeks to overcome at least. some of theaforementioned disadvantages.

SUMMARY OF THE INVENTION

In the following description of the invention, except where the contextrequires otherwise due to express language or necessary implication, thewords “comprise” or variations such as “comprises” or “comprising” areused in an exclusive sense, ie., to specify the presence of statedfeatures, but not to preclude the presence or addition or furtherfeatures in various embodiments of the invention.

Before the invention and preferred embodiments thereof are described, itis to be understood that this invention is not limited to the particularmaterials described, as these may vary. It is also to be understood thatthe terminology used herein is for the purpose of describing particularembodiments only, and is not intended to limit the scope of the presentinvention in any way.

It must also be noted that as used herein, the singular forms of “a”,“an” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms herein have the same meanings as commonly used by oneof ordinary skill in the art to which this invention belongs.

According to one aspect of the present invention there is provided amethod for the recovery of metal values from a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions such as a sulphide ore or concentrate, the process comprising thesteps of:

-   -   (a) providing a feed stream comprising a metal value-bearing        material containing arsenic and/or antimony and a source of        sulphate ions;    -   (b) subjecting the feed stream to oxidative conditions under        elevated temperature and pressure conditions thereby forming a        slurry comprising a metal value-containing leach solution and a        solid residue;    -   (c) separating the metal value-bearing leach solution from the        solid leach residue;    -   (d) recovering the metal value(s) from the metal value-bearing        leach solution; and    -   (e) recovering precious metal values such as gold and/or silver        values, if present, in the solid leach residue by cyanide        leaching.

The slurry from step (b) may be maintained at a temperature in the rangeof from about 70° C. to about 100° C. for a period in the range of fromabout 15 minutes to about 4 hours prior to separating the metalvalue-containing solution from the solid leach residue.

According to another aspect of the present invention there is provided amethod for the recovery of metal values from a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions such as a sulphide ore or concentrate, the process comprising thesteps of:

-   -   (a) providing a feed stream comprising a metal value-bearing        material containing arsenic and/or antimony and a source of        sulphate ions;    -   (b) subjecting the feed stream to oxidative conditions under        elevated temperature and pressure conditions in the presence of        at least one component selected to decrease the effective free        acid concentration during the pressure oxidation step and        promote the formation of pH-stable iron(III) sulphate        conditions, thereby forming a slurry comprising:        -   (i) a metal value-containing leach solution and a solid            residue containing pH-stable iron(III) sulphate products;            and        -   (ii) environmentally stable iron-arsenic and iron-antimony            products, such oxidative conditions comprising the provision            that the Oxygen Reduction Potential (ORP) of the reaction            slurry in at least part of the vessel used for step (b) is            kept below about 425 mV, when measured with a standard            platinum (Pt) electrode against a standard silver/silver            chloride (Ag/AgCl) electrode, and the soluble ferric to            ferrous molar ratio is below about 1:1, and wherein in at            least another part of the vessel the OPR is allowed to            increase above about 425 mV and typically substantially            above about 425 mV so that the soluble ferric to ferrous            molar ratio is above about 1:1 and typically substantially            above about 1:1 to facilitate the precipitation of the            pH-stable iron(III) products and ensure a substantial            proportion, and preferably substantially all, of the            sulphide sulphur is oxidised to sulphate;    -   (c) separating the metal value-bearing leach solution from the        solid leach residue;    -   (d) recovering the metal value(s) from the metal        value-containing leach solution; and    -   (e) recovering precious metals such as gold and/or silver        values, if present, in the solid leach residue by cyanide        leaching.

The vessel of step (b) will typically be a pressure vessel such as anautoclave, and more typically a substantially continuously operatedautoclave.

Up to approximately the first 50% of the total volume of the vessel ofstep (b) may be kept below about 425 mV and typically below about 400mV. Up to approximately 50% of the remaining volume of the vessel ofstep (b) may be allowed to increase above about 425 mV and typicallysubstantially above about 425 mV.

The slurry from step (b) may be maintained at a temperature in. therange of from about 70° C. to about 100° C. for a period in the range offrom about 15 minutes to about 4 hours prior to separating the metalvalue-containing solution from the solid leach residue.

According to another aspect of the present invention there is provided amethod for the recovery of metal values from a metal value-containingfeed material containing arsenic and/or antimony and a source ofsulphate ions such as a sulphide ore or concentrate, the methodcomprising the steps of:

-   -   (a) providing a feed stream comprising a metal value-bearing        material containing arsenic. anchor antimony and a source of        sulphate ions;    -   (b) subjecting the feed stream to oxidative conditions under        elevated temperature and pressure conditions in the presence of        certain iron-containing compounds and/or other chemical agents        selected to decrease the effective free acid concentration        during the pressure oxidation step and promote the formation of        pH-stable iron(III) sulphate conditions, thereby forming a        slurry comprising:        -   (i) a metal value containing leach solution and a solid            residue containing pH-stable iron(III) sulphate products;            and        -   (ii) environmentally stable iron-arsenic and iron-antimony            products, such oxidative conditions including the provision            that the Oxygen Reduction Potential (ORP) of the reaction            slurry in at least part of the vessel used for step (b) is            kept below about 425 mV, when measured with a standard            platinum (Pt) electrode against a standard silver/silver            chloride (Ag/AgCl) electrode, and the soluble ferric to            ferrous molar ratio is below about 1:1, and wherein in at            least another part of the vessel the OPR is allowed to            increase above about 425 mV, and typically substantially            above 425 mV, so that the soluble ferric to ferrous molar            ratio is above about 1:1, and typically substantially above            1:1 to facilitate the precipitation of the. pH-stable            iron(III) products and ensure a substantial proportion, and            preferably substantially all of the sulphide sulphur is            oxidised to sulphate;    -   (c) separating the metal value-bearing leach solution from the        solid leach residue;    -   (d) recovering the metal value(s) from the metal        value-containing leach solution; and    -   (e) recovering precious metal values such as gold ardor silver        values, if present, in the solid leach residue by cyanide        leaching.

The vessel of step (b) will typically be a pressure vessel such as anautoclave, and more typically a substantially continuously operatedautoclave.

Up to approximately the first 50% of the total volume of the vessel ofstep(b) may be kept below about 425 mV and typically below about 40 mV.Up to approximately 50% of the remaining total volume of the vessel ofstep A) may be allowed to increase above about 425 mV and typicallysubstantially above about 425 mV.

The slurry from step (b) may be maintained at a temperature in the rangeof from about 70° C. to about 100° C. for a period in the range of fromabout 15 minutes to about 4 hours prior to separating the metalvalue-containing solution from the solid leach residue.

The terms “pressure oxidation” or “pressure oxidation step” or“oxidative conditions under elevated temperature and pressure” usedherein refer to a high temperature/high pressure leach process operatedunder acidic oxidising conditions.

One particular aspect of the present invention is based upon therealisation that it is possible to adjust the processing conditions suchthat they prevent the formation of insoluble copper-containingprecipitates during the high temperature pressure leaching process toextract metal values such as copper from a metal value-containingmaterial such as a sulphide ore that also contains arsenic and/orantimony.

Another particular aspect of the present invention is based upon therealisation that it is possible to adjust the processing conditions topromote the formation of solid iron(III) sulphate containing-products inthe residue derived from the pressure leaching process that are stableunder the alkaline pH conditions at ambient temperature that are used torecover the gold and/or silver values from the said residue. Forconvenience, this solid iron(III) sulphate containing-product isreferred to as a “pH stable iron(III) sulphate”. Included in the meansof promoting the formation of the pH stable iron(III) sulphate productare means of controlling (decreasing) the free acid generated during thepressure oxidation step by addition of certain additives and/or controlof the slurry ORP in typically about the first 50% of the total volumeof the vessel such as a continuous autoclave used for the pressureoxidation step. This latter means is achieved by limiting the rate ofoxygen injection into about the first 50% of the total volume of thecontinuous autoclave.

The result of the correct selection of the high temperature/highpressure leaching conditions for treating metal value-bearing materialscontaining arsenic and/or antimony is that the majority of the arsenicand/or antimony reports to a solid residue as an environmentally stablemixed iron-arsenic and/or iron-antimony solid species mixed with pHstable iron(III) sulphate products. In addition, copper losses to theresidue are minimised by prevention of precipitation of acopper-iron-sulphate-arsenate, while cyanidation of the gold and/orsilver content of the leach residue is enhanced because of the promotionof precipitation of pH stable iron(III) sulphate products such asjarosite-type minerals rather than basic iron sulphate.

The present invention is accordingly concerned with the development ofeconomically viable conditions that can at least partially achieve oneor more of (a) minimizing copper losses to the leach residue, (b)ensuring that the arsenic and/or antimony components of the feedmaterial report to the residue in an environmentally stable form, and(c) preventing the formation of solid residues that break down duringthe gold and/or silver cyanidation step and a concomitant increase inlime and cyanide consumption in the case where the initial feed materialcontains recoverable gold and/or silver.

Preferably, the metal value-bearing material containing arsenic and/orantimony is a copper-bearing material containing arsenic and/orantimony, in particular a copper sulphide containing arsenic and/orantimony, and even more particularly a mixed copper-gold sulphidecontaining arsenic and/or antimony. Typically the metal value-containingmaterial is an ore or concentrate that contains arsenic and/or antimony,and include but is not limited to:

-   -   (a) an ore or concentrate that contains recoverable base and        other metals including but not limited to copper, nickel,        cobalt, zinc, and the platinum group metals;    -   (b) an ore or concentrate that contains recoverable precious        metals, especially gold and silver,    -   (c) an ore or concentrate that contains recoverable base and        other metals including but not limited to copper, nickel,        cobalt, zinc and the platinum group metals, as well as precious        metals, especially gold and silver.

Typically, the pH stable iron(III) sulphate product formed in theabovementioned pressure leach step is composed of one or morejarosite-type minerals, such as hydronium, sodium, potassium or ammoniumjarosite. In one preferred embodiment of the present invention, the pHsable iron(III) sulphate product is hydronium and/or sodium jarosite.

While it is common for a metal value-bearing material containing arsenicand/or antimony such as a copper sulphide ore or concentrate containingarsenic and/or antimony to also contain at least trace amounts of ironcompounds, the inventors have advantageously found that the presence ofadditional iron compounds in the feed material subjected to the pressureleaching process also promotes the formation of copper-free secondaryferric sulphate minerals that also contain arsenic and/or antimony.Preferably, the molar ratio of Fe:(As+Sb) in the feed material to step(b) of the preferred embodiments described above is greater than about1:1, and. more preferably greater than about 2:1. Thus by ensuring thatthe Fe:(As+Sb) molar ratio in the feed material to step (b) is greaterthan about 1:1 and preferably greater than about 2:1, the bulk of thearsenic and/or antimony in the feed material reports to the residue asan environmentally stable iron-arsenate and/or iron-antimonate phase,rather than as a copper-iron-sulphate-arsenate/antimonate.

The inventors have found that the iron compounds suitable for theabovementioned modification to the Fe:(As+Sb) molar ratio in the feedmaterial are such compounds that are readily solubilised under theacidic high temperature/high pressure leach conditions of the invention.The particle size of the suitable iron compounds will typically be suchthat the solubilisation kinetics are compatible with the retention timeif the high temperature/hi pressure leach stage.

Provided that the requirement for rapid solubilisation under the hightemperature/high pressure leach conditions is met, the chemical valencyof the iron compounds added to the feed material to adjust theFe:(As+Sb) molar ratio to the required level is not thought to becritical. This is because, under the operating conditions of the hightemperature/high pressure leach step (b), substantially all ferrous[Fed(II)] will be rapidly oxidised to the ferric [Fe(III)] state. Inother words, the iron compounds may be ferrous or ferric compounds, ormixed ferrous/ferric compounds. However, it is preferred that the ironcompounds are in the ferric state since this reduces the Len consumptionduring the high temperature/high pressure leach step.

In one preferred embodiment of the invention, the iron compounds arederived from pyrite, in particular calcined pyrite produced underconditions that favour the formation of FeS, FeO, FeO₄ or gamma-Fe₂O₃over the formation of alpha-Fe₂O₃, since the former iron compounds aremore readily solubilised compared with the latter iron compound.

During the high temperature/high pressure leach step there are manycompeting chemical reactions relating to the formation and precipitationof different iron-containing species, such as, for example, basic ferricsulphates, hematite, and jarosite. Promotion of precipitation ofjarosite and/or hematite aver basic iron sulphate is favoured by thepresence of suitable reactions that decrease the effective concentrationof free acid generated during the high temperature/high pressure leachstep.

In one preferred embodiment of the invention, the chemical agents addedto the feed material being subjected to the high temperature/highpressure leach step comprise metal salts which directly participate inthe formation of jarosite-type compounds, in particular soluble alkalimetal ion salts such as those of sodium or potassium, and ammoniumsalts, all of which form stable jarosite-type minerals of the generalformula MFe₃(SO₄)₂(OH)₆ where M=Na, K and NH₄, respectively. Theformation of these jarosite-type minerals deceases the effectiveconcentration of free acid under the prevailing high temperature/highpressure leach conditions. The addition of such soluble alkali metal ionsalts also increases the temperature at which jarosite-type mineralstend to form in preference to basic iron sulphate type minerals duringthe pressure oxidation process at any given acid concentration. Theability to operate at higher temperatures while promoting the formationof pH-stable iron(III) sulphate products over basic iron sulphate typeminerals provides economic advantages in the form of enhanced leachingreaction kinetics and shorter required residence (retention) times.

In another preferred embodiment of the invention, the chemical agentsalso comprise soluble sulphate salts whose cations are merely spectatorions and as such do not participate in any precipitation reactions. Thepreferred chemical reagents particularly include magnesium and/or zinc.Addition of a suitable soluble sulphate increases the concentration ofthe bisulphate ion present in the high temperature/high pressure leachslurry and decreases the effective concentration of free acid attemperature from that which would otherwise be experienced at a givenfeed solids composition and concentration (% solids). The solublesulphate salts may be added directly to the high temperature/highpressure leach step or generated by reacting carbonate and/or hydroxidesalts of magnesium and/or zinc in the high temperature/high pressureleach step. In another preferred embodiment of the invention, thesoluble zinc salt may be introduced by the leaching of zinc sulphideminerals that may be present in the feed material.

In a further preferred embodiment of the invention, the chemicalreagents may also comprise bases or carbonates, in particular limestoneor lime, which directly consume acid and decrease the effectiveconcentration of free acid in the high temperature/high pressure leachstep.

In the case of copper sulphides containing arsenic and/or antimony,copper dissolution in the high temperature/high pressure leach step isoptimised by addition of iron compounds to the reaction vessel,typically an autoclave, in sufficient quantities to favour precipitationof environmentally stable secondary iron-arsenate and/or iron-antimonateand/or iron-arsenic-sulphate and/or iron-antimonate-sulphate phaseswithin the autoclave rather than the precipitation of copper-containingarsenate-antimonate residues, thereby limiting the copper content of theleach residue and maximising the soluble copper content of the resultantliquid stream available for copper recovery by a combination of solventextraction and electrowinning or by means of another suitable recoverymethod.

By means of limiting the copper content of the solid leach residue andefficient separation of the soluble copper from the solid leach residue,the economics of gold and/or silver recovery from the leach residue bycyanidation is enhanced as the extent of the reaction between copper andthe cyanide leachant is significantly reduced, thereby lowering theoverall cyanide consumption.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with various aspects of the present invention, a metalvalue-bearing material containing arsenic and/or antimony that alsoconstitutes a source of sulphate ions is provided for processing. Themetal value-bearing material may be an ore, concentrate, or any othermaterial from which metal values, in particular copper and gold and/orsilver values, may be recovered. The invention is equally applicable toother metal value-bearing materials containing arsenic and/or antimonysuch as ores and concentrates containing other valuable metals such asnickel, cobalt, zinc and the platinum-group metals.

For convenience, however, the description of the preferred embodimentsof the invention is restricted to copper-containing materials that alsocontain arsenic and/or antimony. The copper-containing material ispreferably a copper sulphide ore or concentrate that contains arsenicand/or antimony, and particularly applies to ores and/or concentratesthat contain tennantite (Cu₁₂As₄S₁₃), enargite (Cu₃AsS₄) andtetrahedrite (Cu₁₂Sb₄S₁₃), and to other ores or concentrates containingcopper sulphide minerals such as, for example, chalcopyrite (CuFeS₂),chalcocite (Cu₂S), bornite (Cu₅FeS₄) and covellite (CuS), whencontaminated with arsenic- and/or antimony-bearing material.

Geologically gold and/or silver are frequently associated with metalsulphide ores such as, for example, pyrite, chalcopyrite, galena,arsenopyrite and stibnite. Gold and/or silver are also often present insulphide concentrates produced from such ores. Accordingly, a preferredembodiment of the present invention is particularly advantageous inconnection with the recovery of copper and gold and/or silver from mixedgold/silver/copper ores or concentrates containing arsenic and/orantimony. Thus, the metal value-bearing material is preferably a mixedgold/silver/copper ore or concentrate containing arsenic and/orantimony. Typically the mixed gold/silver/copper ore is atennantite-enargite-calcopyrite-pyrite ore.

The metal value-bearing material typically undergoes comminution,flotation, blending and/or slurry formation, as well as chemical and/orphysical conditioning to afford a feed stream which, in turn, issubjected to a high temperature/high pressure oxidative leach step and aseries of downstream unit stages to afford recovery of the containedmetal values.

The specific conditions applicable to the comminution, flotation andconditioning stages are determined by the chemical and physicalproperties of the metal value-bearing ore material. As a general rule,these specific conditions are designed to yield a concentrate thatoptimises recovery versus grade. These specific conditions do not have adirect bearing on the application of the preferred embodiments of thepresent invention. As such, the present invention is primarily concernedwith the treatment of a dewatered concentrate exiting the comminution,flotation and conditioning circuits.

After the metal value-bearing concentrate stream has been suitablyprepared as a slurry, the slurry is fed to an agitated pressure vessel,preferably an autoclave, and subjected to pressure oxidation. Typicallythe high temperature/high pressure leaching process is carried out at atemperature in the range of from about 180° C. to about 250° C.,preferably from about 190° C. to about 230° C. The optimum temperaturedepends on many factors including, but not limited to, the mineralogicalcomposition of the feed, the sulphide sulphur content of the feed, theparticle size distribution of the feed, and the pulp density. As ageneral rule, the higher temperatures in the above ranges provide forshorter retention times and/or a reduction and/or elimination of theneed for regrinding of the feed material prior to the hightemperature/high pressure leach step.

The high temperature/high pressure leaching process is typically carriedout at a total pressure sufficiently high to provide an oxygen partialpressure inside the autoclave of between about 100 kPa and about 1500kPa, preferably in the range of from about 400 kPa to about 1000 kPa,taking into account the partial pressure of steam and otherton-condensable gases within the autoclave such as nitrogen and carbondioxide. Oxygen is typically delivered to the autoclave by bottom entryspargers entering beneath the autoclave agitators at a pressure abovethat inside the autoclave. The autoclave agitators are designed tomaximise oxygen mass transfer from the gas phase to the feed slurry.

In one of the preferred embodiments of the present invention, it hasbeen found advantageous to control the Oxygen Reduction Potential (ORP)of the slurry in the first compartment of the autoclave and morepreferably in approximately the first 50% of the total autoclave volume,to a value below about 425 mV, and more preferably below about 400 mV,when measured against a standard platinum (Pt) electrode against astandard silver/silver chloride (Ag/AgCl) reference electrode. In thisinstance, the ORP is recorded within one hour using a filtered slurrysample withdrawn from the autoclave that had been rapidly cooled toambient temperature. Control of the ORP is achieved by limiting the rateof oxygen injection into the first compartment and more preferablyapproximately the first 50% of the total autoclave. volume. In theremaining autoclave compartments and/or approximately the second 50% ofthe total autoclave volume the ORP is allowed to increase above about500 mV by increasing the rate of oxygen injection into the autoclave.The inventors have found that control of the ORP in the above mannerpermits regulation of the oxidation of ferrous iron to the ferric stateas the slurry moves through the autoclave and assists in the generationof solid pH stable iron(III) sulphate products.

The high temperature/high pressure leach step is typically conductedover a period of from about 20 minutes to about 4 hours, and morepreferably to about 2 hours, with higher operating temperatures and afiner feed particle size facilitating shorter reaction times.

Under the high temperature/high pressure leaching process conditions,solid metal sulphide minerals within the feed material are oxidised tothe corresponding soluble metal sulphates. That is, the metal values arereleased into solution. The actual oxidation/dissolution reactions foreach metal sulphide mineral are a reflection of the chemical compositionof that mineral as well as the temperature and free acidity of the leachslurry, but the overall reaction can be simplified as shown in reaction(1).

MS(solid)+2O₂(gas)→MSO₄(solution)  (1)

The arsenic and antimony components of the feed material are oxidized tothe arsenate (ASO₄ ³) and antimonite (SbO₄ ³⁻) species, respectively.

Some of the solubilised metal values then re-precipitate within theautoclave and report to the solid phase component of the autoclaveslurry as metal oxides and/or metal mixed hydroxyl-sulphates and/ormetal-sulphate-arsenate-antimonate species.

Iron may report to the solid phase component of the autoclave slurry asone or more different iron-containing compounds during the hightemperature/high pressure leach process, the identity of such phasesbeing determined by a specific set of operating conditions. For example,the formation of basic iron sulphate is favoured by high operatingtemperatures and high free acid conditions. Under such conditions, theoxidation of pyrite (FeS₂), a significant component of many metalsulphide concentrates, can be represented by reaction (2).

4FeS₂+15O₂+6H₂O→4Fe(OH)SO₄+4H₂SO₄  (2)

The reaction of pyrite to form hematite (alpha Fe₂O₃) is favoured byhigh temperatures and low free acidity concentrations according toreaction (3).

4FeS₂+17O₂+8H₂O→2Fe₂O₃+8H₂SO₄  (3)

The formation of jarosite is favoured by low operating temperatures andthe presence of cations such as Na⁺, K⁺ or NH₄ ⁺, according to reaction(4) where M=Na, K or NH₄.

12FeS₂+45O₂+30H₂O+2M₂SO₄→4MFe₃(SO₄)₂(OH)₆+18H₂SO₄  (4)

Hydronium jarosite, in which M=H₃O⁺, the hydronium ion of free acid,takes the place of Na, K or NH₄ is also favoured by low operatingtemperatures in the absence of such cations.

Arsenate and antimonate species formed by the oxidation of the arsenicand antimony components of the feed material may precipitate as therespective iron(III) arsenate and iron(III) antimonate phases, but mayalso substitute for sulphate in, for example, the jarosite phase. Theprecipitation of arsenate as hydrated iron(III) arsenate, FeAsO₄2H₂O,also known as scorodite, and the partial replacement of sulphate byarsenate in various jarosite phases is well documented in the scientificliterature. Jarosite is sometimes referred as a scavenger for botharsenate and antimonate. The formation of hydrated iron(III) arsenateand/or arsenic-containing jarosite materials in the present invention isof considerable environmental benefit since these materials are known tobe environmentally stable and can be safely discharged into and storedin conventional residue storage impoundments.

Under typical prior art operating conditions for the hightemperature/high pressure leaching of mixed copper/gold metal sulphideconcentrates containing arsenic and/or antimony, formation of basic ironsulphate and hematite are favoured. The basic iron sulphate and hematitereport to the solid residue resulting from the high temperature/highpressure leach process. when the solid residue is washed, repulped andthen subjected to cyanidation in order to extract the gold and/or silvervalues therein, there is an uneconomically high consumption of lime andcyanide. This is because the lime reacts directly with the basic ironsulphate during the adjustment of the pH to a value of 10 or higher thatis required for the gold and/silver cyanidation step.

In the present invention, additional iron compounds are added to thefeed material to the high temperature/high pressure leach step in orderto promote the formation of jarosite rather than basic iron sulphate.Under these conditions the jarosite phase acts as an efficient scavengerfor any soluble arsenate and/or antimonate formed during the pressureoxidation reactions. Moreover, the jarosite phase does not itself reactwith lime when the gold and/or silver are recovered from the leachresidue by cyanidation.

Preferably, the total iron content of the feed material to the hightemperature/high pressure leach process is such that the molar ratio ofFe:(As+Sb) is greater than about 2:1 and more preferably at least about4:1. Apart from facilitating the formation of arsenic- andantimony-containing jarosite phases which do not react with lime duringcyanidation, the high Fe:(As+Sb) molar ratio reduces and/or prevents theformation and precipitation of a mixedcopper-iron-arsenate-antimonate-sulphate phase

The iron compounds added to the metal value-bearing feed material inorder to adjust the molar ratio of Fe:(As+Sb) to the desired level areof a mineral/chemical composition and particle size such that they arereadily solubilised under the acidic high temperature/high pressureleach conditions.

The valency of the iron in the iron compounds is not thought to becritical because under the operating conditions of the hightemperature/high pressure leach process, substantially all iron(II) willbe oxidised to iron(III). In other words, the iron compounds may beferrous or ferric compounds or mired ferrous/ferric compounds, providedthat they are soluble under the high temperature/high pressure leachconditions. However, it is preferred that the iron compounds arepre-treated to maximise the ferric content and minimise any sulphidecontent in order to lower the overall oxen consumption required duringthe high temperature/high pressure leach step.

In a preferred embodiment of the present invention the iron compoundsare derived from pyrite, in particular calcined pyrite produced byoxidative conditions with the calciner operated in such a fashion as toproduce a calcined pyrite with a significant portion of the iron presentin a form readily capable of being solubilised in the autoclave underthe high temperature/high pressure conditions, such as for example, FeS,FeO, Fe₃O₄ or gamma-Fe₂O₃, rather than alpha-Fe₂O₃ produced in aconventional pyrite roaster, or the higher sulphide containing FeS₂ oruncalcined pyrite.

In a further preferred embodiment of the present invention, the ironcompounds may be sourced from recycled process solutions containing ironsulphate, preferably in the ferric form, although the process solutionsmay also carry minor amounts of ferrous iron as well. Alternatively, theiron compounds may be iron-containing precipitates from various otherparts of the overall process, such as the iron-containing precipitateproduced during minor impurity removal ahead of or subsequent to metalvalue recovery steps such as copper recovery by a combination of solventextraction and electrowinning.

The iron compounds may be mixed with the metal value-bearing feed streambefore it is transferred to the high temperature/high pressure autoclaveleach vessel, or the iron compounds may be separately transferred to theautoclave before or after introduction of the feed stream to theautoclave.

One of the preferred embodiments of the present invention incorporatesthe addition of specific chemical agents which decrease the effective.concentration of free acid generated during the high temperature/highpressure leaching process thereby affording the precipitation of pHstable iron(III) sulphate compounds and avoiding the precipitation of abasic ferric sulphate. One group of chemical agents includes metal saltsthat directly participate in the formation of jarosite-type compounds,in particular sodium, potassium and ammonium jarosites. Such metal saltsinclude soluble alkali metal (sodium and potassium) and ammoniumsulphate. Typically the molar ratio of the added metal salt per mole ofiron present in the feed should be at least 1:3 and preferably at leastabout 1:2, that is, an excess of metal salt above the stoichiometricrequirement.

Another group of chemical agents that have the ability to decrease theeffective concentration of free acid generated during the hightemperature/high pressure leaching process comprise soluble sulphatesalts whose cations are merely spectator ions and which do notparticipate in any precipitation reactions. Addition of soluble sulphateincreases the concentration of the bisulphate ion present at theoperating high temperature and decreases the effective concentration offree acid that would otherwise be expected at the given temperature,feeds solids composition and pulp density.

The soluble sulphate salts may be directly added to the hightemperature/high pressure leaching step or generated by reactingcarbonate or hydroxide salts of the appropriate metals. The inventorshave established that the appropriate metal sulphate salts include thoseof magnesium and zinc. Typically, magnesium is added as magnesiumcarbonate (magnesite), magnesium oxide, dolomite, or mixtures thereof.

The soluble sulphate salts, once added to or generated by the overallprocess, may be conveniently recycled in process water used for feedpreparation and/or autoclave quench water once the copper or otherdissolved metal values have been recovered from the leach solution.

The chemical agents may also comprise carbonates and other bases, inparticular limestone and lime, which directly consume acid and decreasethe effective concentration of free acid during the hightemperature/high pressure leach process. Typically, bases are added inan amount necessary to yield less than about 60 g/L sulphuric acid insolution in the product from the high temperature/high pressure leachstep, as measured by titration of slurry samples at ambient temperature.

The chemical agents may be mixed with the feed stream before it istransferred to die autoclave for the high temperature/high pressureleach step, or the chemical agents may be separately transfer to theautoclave before or after introduction of the feed stream to theautoclave.

During the high temperature/high pressure leach step metal values, inparticular copper, may be solubilised to form a metal value-containingsolution. It is envisaged that the metal values will be recovered fromthe metal value-containing solution by well understood methods andtechniques. For example, where the metal value is copper, copper istypically recovered from the copper-containing solution by a combinationof solvent extraction and electrowinning. However, other metal recoveryprocesses such as cementation or precipitation of an intermediateproduct such as a hydroxide or sulphide could be employed. In apreferred embodiment of the present invention it has been found to beadvantageous to maintain the slurry discharged from the autoclave at atemperature above about 70° C., and preferably in the range of about85-100° C. for a period in the range of from about 15 minutes to about 4hours in an agitated tank or series of tanks before it is subsequentlycooled to ambient temperature and subjected to solid/liquid separationby counter current decantation and thickening ahead of the metalrecovery from solution and gold and/or silver recovery from the solidresidue. This compares with prior art that incorporates rapid cooling ofthe autoclave discharge slurry to ambient temperature by means of aseries of flash vessels and subsequent solid/liquid separation processedgenerally conducted below about 70° C. The advantage of this slowcooling or digestion-conditioning step disclosed in the presentinvention relates to the fact that any remaining basic ferric sulphateand/or copper-iron-sulphate-arsenate-antimonate in the leach slurry willbe converted into a pH stable iron(E) sulphate and/or redissolve, whichin the case of copper-iron-sulphate-arsenate-antimonate will releasesoluble copper, respectively. By this means, the lime consumptionrequired and, in the case of copper-containing feed materials, thecyanide consumption required for gold and/or silver cyanidation shouldbe reduced, while any copper losses to the solid leach residue shouldalso be reduced.

Precious metal values such as gold and/or silver values contained in thefeed material will report to the solid residue formed during the hightemperature/high pressure leach process. It is envisaged that the goldand/or silver values will be recovered from the solid residue by washingto remove entrained acid and soluble metal values, repulping andtreating the consequent slurry by a combination of conventionalcyanidation, activated carbon, stripping, electrowinning and smeltingtechniques.

By application of the preferred embodiments of the present invention,copper recoveries in excess of 95% and lime consumption of less than 15kg/t of solid residue can be expected form a wide range of copper/goldsulphide ores and concentrates that also contain appreciable arsenicand/or antimony contents.

In summary, the advantages of the present invention compared with priorart include but are not limited to the following:

-   -   (a) enhanced recovery of metal values, typically copper and        gold, by preventing the co-precipitation of metal values in the        solid residue discharged from the high temperature/high pressure        leach step;    -   (b) prevention of the formation of unstable basic iron sulphate        species in the solid residue discharged from the high        temperature/high pressure leach step that consume excessive lime        during the recovery of the gold and/or silver by cyanidation;        and    -   (c) generation of arsenic- and/or antimony-containing residues        that can be stored in conventional residue storage impoundments        without causing unacceptable environmental outcomes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention are now described byreference to the following example. The process conditions reflectedtherein are intended to exemplify various aspects of the invention, andare not intended to limit the scope of the claimed invention. Numerousvariations and modifications will suggest themselves to persons skilledin the relevant art, in addition to those already described, withoutdeparting from the basic inventive steps. All such variations andmodifications are to be considered within the scope of the presentinvention, the nature of which is to be determined from the foregoingdescription.

EXAMPLE

This example outlines the general scope of the preferred embodiments ofthe present invention as applied to the continuous processing of arun-of-mine tennantite-enargite-chalcopyrite-pyrite ore containing onaverage 1.5% Cu and 3.8 g/t gold derived in from the Chelopech(Bulgaria) resource. A simplified flowsheet of one preferred embodimentof the present invention is shown in FIG. 1.

After crushing and grinding, a copper concentrate typically containing15.5% Cu, 24.8% Fe, 38.1% S, 4.7% As and 30 g/t Au is produced byrougher, scavenger and cleaner flotation banks using the appropriateflotation reagent regime. The copper concentrate is directed to a copperconcentrate dewatering circuit where the free moisture is reduced toabout 10%.

The copper concentrate is repulped in neutral barren solution (NBS)derived from the downstream copper recovery circuit (solvent extractionand electrowinning) that typically contains about 42 g/L MgSO₄ and 15 gZnSO₄ at pH 8.5, prior to regrinding to a P₈₀ of 25 micron. The regroundconcentrate is thickened to approximately 55% solids and transferred tothe agitated autoclave feed tank. To this tank are added controlledamounts of underflows from the final impurity (IR) stages of the SXraffinate and mine water treatment circuits, as well as a limestoneslurry sufficient to achieve the desired carbonate-sulphur ratio in thefeed. The relative amounts of limestone slurry and impurity removalunderflow added to the reground concentrate are controlled to ensurethat the free acidity and Fe.(As+Sb) molar ratio of the feed slurry aresufficient to prevent the precipitation of unstable basic ferricsulphate and copper-iron-sulphate-arsenate phases in the autoclavedischarge slurry. The solid component of the blended regroundconcentrate, limestone. and impurity removal slurry typically containsabout 8.5% Cu, 13.1% Fe, 24.5% S, 2.7% As and 8.8 g/t Au, which ispumped into the high temperature/high pressure leach autoclave as a 45%solids slurry.

The combined slurry is directed to the first component of amulti-compartment high pressure autoclave fitted with a plurality ofagitators by means of a centrifugal pump feeding a positivedisplacement, piston driven diaphragm pump at an operating pressure ofover the steam saturation pressure at the operating temperature, whichwill generally be over 2000 kPa.

High pressure steam is supplied to the autoclave for initial heat-up andon as-needed basis.

Each compartment of the autoclave is fitted with a quench water systemby which a controlled flow of quench water, typically neutral barrensolution (NBS), can be directly injected into each compartment such thatthe desired operating temperature, typically in the range of from about190° C. to about 230° C., is continuously maintained. The use of NBS asquench water assists with maintaining the overall process flowsheetwater balance, and since it also contains appreciable magnesium and zincsulphate contents, also assists with the control of the autoclave slurrychemistry.

Oxygen at 94% or greater purity is delivered from a cryogenic oxygenplant to the autoclave by bottom entry spargers entering beneath each ofthe autoclave agitators at a pressure greater than about 2000 kPa. Thebottom impeller on the agitators is of the Rushton turbine design inorder to maximize oxygen mass transfer to the feed slurry.

The rate of oxygen injection into the first 50% of the total autoclavevolume is controlled such that the Oxygen Reduction Potential (ORP), aspreviously defined and measured, is maintained at or below about 400 mV.The rate of oxygen injection into the remaining 50% of the totalautoclave volume. is increased so that the ORP increases to above about500 mV to enhance the oxidation of ferrous iron to the ferric state.

Following the required retention time, typically about 60-80 minutes,the processed slurry is discharged from the autoclave via a single stageflash vessel at approximately 100° C. Flashed slurry flows by gravitythrough two agitated discharge tanks connected in series with a totalretention time of about 2 hours, where the temperature is maintained at85-100° C. From there the conditioned autoclave slurry is subjected tosolid/liquid separation via a series of five conventional countercurrent thickeners.

The thickened underflow is washed to remove entrained leach solution,washed and the resultant cake forwarded to a conventional goldcyanidation circuit.

The final thickener overflow contains the dissolved content of the feedand is directed to a primary neutralisation (PN) circuit as pregnantleach solution (PLS). As the PLS contains a relatively high sulphuricacid concentration, typically 30-60 g/L, excess acid is neutralised byaddition of a limestone slurry to achieve a final PLS free acidity ofabout 2-5 g/L (pH approximately 1.5). After solid/liquid separation toremove precipitated solids, principally gypsum, the neutralised PLS isclarified before the copper is recovered by conventional solventextraction and electrowinning techniques.

The raffinate from the solvent extraction circuit is then subjected toan impurity removal (IR) step by addition of limestone and limeslurries. After removal of the precipitated solids, which are recycledto the autoclave feed slurry preparation circuit, the clarified neutralbarren solution (NBS) is used in a variety of appropriate duties notedabove, including repulping of the incoming dewatered concentrate and asautoclave quench water.

Modifications and improvements to the invention will be readily apparentto those skilled in the art. Such modifications and improvements areintended to be within the scope of this invention.

1. A method for the recovery of metal values from a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions, comprising the steps of: (a) providing a feed stream comprising ametal value-bearing material containing arsenic and/or antimony and asource of sulphate ions; (b) subjecting the feed stream to oxidativeconditions under elevated temperature and pressure conditions therebyforming a slurry comprising a metal value-containing leach solution anda solid residue; (c) separating the metal value-bearing leach solutionfrom the solid leach residue; (d) recovering the metal value(s) from themetal value-bearing leach solution; and (e) recovering any preciousmetal values in the solid leach residue by cyanide leaching.
 2. Themethod according to claim 1, wherein the slurry from step (b) ismaintained at a temperature in the range of from about 70° C. to about100° C. for a period in the range of from about 15 minutes to about 4hours prior to separating the metal value-containing solution from thesolid leach residue.
 3. A method for the recovery of metal values from ametal value-bearing material containing arsenic and/or antimony and asource of sulphate ions, comprising the steps of: (a) providing a feedstream comprising a metal value-bearing material containing arsenicand/or antimony and a source of sulphate ions; (b) subjecting the feedstream to oxidative conditions under elevated temperature and pressureconditions in the presence of at least one component selected todecrease the effective free acid concentration during the pressureoxidation step and promote the formation of pH-stable iron(IH) sulphateproducts, thereby forming a slurry comprising: (i) a metalvalue-containing leach solution and a solid residue containing pH-stableiron (Ill) sulphate products; and (ii) environmentally stableiron-arsenic and iron-antimony products, (c) separating the metalvalue-bearing leach solution from the solid leach residue; (d)recovering the metal value(s) from the metal value-containing leachsolution; and (e) recovering any precious metals in the solid leachresidue by cyanide leaching.
 4. The method according to claim 3, whereinthe oxidation conditions in the vessel used in step (b) provide theslurry, in at least a first part of the vessel, with an Oxygen ReductionPotential (ORP) of below about 425 mV, when measured with a standardplatinum (Pt) electrode against a standard silver/silver chloride(Ag/AgCl) electrode, and a soluble ferric to ferrous molar ratio ofbelow about 1:1, and wherein the oxidation conditions provide theslurry, in at least a second part of the vessel, with an OPR of aboveabout 425 mV and the soluble ferric to ferrous molar ratio of aboveabout 1:1, to facilitate the precipitation of the pH-stable iran(HI)products and oxidation of the sulphide sulphur to sulphate.
 5. Themethod according to claim 4, wherein the ORP in the reaction slurry insaid first part of the vessel is below about 400 mV.
 6. The methodaccording to claim 4, wherein said first part of the vessel encompassesup to about 50% of the total volume of the vessel used in step (b). 7.The method according to claim 4, wherein said second part of the vesselencompasses up to about 50% of the total volume of the vessel used instep (b).
 8. The method according to claim 4, wherein the oxidationconditions are controlled by limiting the rate of oxygen injection intothe first and/or second part of the vessel.
 9. The method according toclaim 4, wherein the vessel of step (b) is a pressure vessel, preferablyan autoclave, and more preferably, a substantially continuously operatedautoclave.
 10. The method according to claim 3, wherein the slurry fromstep (b) is maintained at a temperature in the range of from about 70°C. to about 100° C. for a period in the range of from about 15 minutesto about 4 hours prior to separating the metal value-containing solutionfrom the solid leach residue.
 11. A method for the recovery of metalvalues from a metal value-containing feed material containing arsenicand/or antimony and a source of sulphate ions, the method comprising thesteps of: (a) providing a feed stream comprising a metal value-bearingmaterial containing arsenic and/or antimony and a source of sulphateions; (b) subjecting the feed stream to oxidative conditions underelevated temperature and pressure conditions in the presence of certainiron-containing compounds and/or other chemical agents selected todecrease the effective free acid concentration during the pressureoxidation step and promote the formation of pH-stable iron(III) sulphateproducts, thereby forming a slurry comprising: (i) a metalvalue-containing leach solution and a solid residue containing pH-stableiron (III) sulphate products; and (ii) environmentally stableiron-arsenic and iron-antimony products; (c) separating the metalvalue-bearing leach solution from the solid leach residue; (d)recovering the metal value (s) from the metal value-containing leachsolution; and (e) recovering any precious metal values in the solidleach residue by cyanide leaching.
 12. The method according to claim 11,wherein the oxidation conditions in the vessel used in step (b) providethe slurry, in at least a first part of the vessel, with an OxygenReduction Potential (ORP) of below about 425 mV, when measured with astandard platinum (Pt) electrode against a standard silver/silverchloride (Ag/AgCl) electrode, and a soluble ferric to ferrous molarratio of below about 1:1, and wherein the oxidation conditions providethe slurry, in at least a second part of the vessel, with an OPR ofabove about 425 mV and the soluble ferric to ferrous molar ratio ofabove about 1:1, to facilitate the precipitation of the pH-stableiron(III) products and oxidation of the sulphide sulphur to sulphate.13. The method according to claim 12, wherein the ORP in the reactionslurry in said first part of the vessel is below about 400 mV.
 14. Themethod according to claim 12, wherein said first part of the vesselencompasses up to about 50% of the total volume of the vessel used instep (b).
 15. The method according to claim 12, wherein said second partof the vessel encompasses up to about 50% of the total volume of thevessel used in step (b).
 16. The method according to claim 12, whereinthe oxidation conditions are controlled by limiting the rate of oxygeninjection into the first and/or second part of the vessel.
 17. Themethod according to claim 12, wherein the vessel of step (b) is apressure vessel, preferably an autoclave, and more preferably, asubstantially continuously operated autoclave.
 18. The method accordingto claim 11, wherein the slurry from step (b) is maintained at atemperature in the range of from about 70° C. to about 100° C. for aperiod in the range of from about 15 minutes to about 4 hours prior toseparating the metal value-containing solution from the solid leachresidue.
 19. The method according to claim 11, wherein the chemicalagents added to the material in the feed stream include metal salts,preferably soluble alkali metal ion salts, more preferably sodium,potassium and ammonium salts.
 20. The method according to claim 11,wherein the chemical agents added to the material in the feed streaminclude a source of soluble sulphate salts, preferably magnesium and/orzinc sulphate.
 21. The method according to claim 20, wherein the sourceof soluble sulphate salts include carbonate and/or hydroxide salts ofmagnesium and/or zinc formed in situ under the oxidative conditions ofstep (b) or by the leaching of zinc sulphide minerals present in thematerial in the feed stream.
 22. The method according to claim 11,wherein the chemical agents added to the material in the feed streaminclude a base and/or carbonate, preferably limestone or lime.
 23. Themethod according to claim 3, wherein the pH stable iron(IH) sulphateproducts formed are composed of one or more jarosite-type mineralsincluding hydronium, sodium, potassium or ammonium jarosite, preferablyhydronium and/or sodium jarosite.
 24. The method according to claim 1,wherein the metal value-bearing material containing arsenic and/orantimony is a copper-bearing material containing arsenic and/orantimony, preferably a copper sulphide containing arsenic and/orantimony, and more preferably a mixed copper-gold sulphide containingarsenic and/or antimony.
 25. The method according to claim 1, whereinthe metal value-containing material is an ore or ore concentrate thatcontains arsenic and/or antimony, and includes one or more recoverablemetals selected from the group consisting of copper, nickel, cobalt,zinc, palladium and platinum.
 26. The method according to claim 1,wherein the metal value-containing material is an ore or ore concentratethat includes one or more recoverable precious metals, preferably goldand silver.
 27. The method according to claim 1, wherein die material inthe feed stream includes iron compounds, preferably iron (IU) compounds.28. The method according to claim 27, wherein the molar ratio ofFe:(As+Sb) in the material in the feed stream in step (b) is greaterthan about 1:1, and preferably greater than about 2:1.
 29. The methodaccording to claim 27, wherein the iron compounds are derived frompyrite, preferably calcined pyrite produced under conditions that favourthe formation of more solubilzable forms of iron compounds includingFeS, FeO, Fe₃O₄ or gamma-Fe₂O_(j) over the formation of alpha-Fe₂O₃. 30.The method according to claim 1, wherein prior to the step of recoveringthe metal value(s) from the metal value-containing leach solution, thepH is reduced to a pH of less than about pH2.